// $Id: PacketEncoding.cs,v 1.1.1.1 2007/07/03 10:15:18 tamirgal Exp $

/// <summary>************************************************************************
/// Copyright (C) 2001, Patrick Charles and Jonas Lehmann                   *
/// Distributed under the Mozilla Public License                            *
/// http://www.mozilla.org/NPL/MPL-1.1.txt                                *
/// *************************************************************************
/// </summary>
using System;
namespace Tamir.IPLib.Packets
{
	
	
	/// <summary> Packet encoding.
	/// <p>
	/// Contains utility methods for decoding generic packets.
	/// 
	/// </summary>
	/// <author>  Patrick Charles and Jonas Lehmann
	/// </author>
	/// <version>  $Revision: 1.1.1.1 $
	/// </version>
	/// <lastModifiedBy>  $Author: tamirgal $ </lastModifiedBy>
	/// <lastModifiedAt>  $Date: 2007/07/03 10:15:18 $ </lastModifiedAt>
	public class PacketEncoding
	{
		
		// create an empty array to return whenever we need to return an, er,
		// empty array. this should be okay, because how can you mutate a 
		// 0-length array?
		//UPGRADE_NOTE: Final was removed from the declaration of 'EMPTY_BYTE_ARRAY '. "ms-help://MS.VSCC.v80/dv_commoner/local/redirect.htm?index='!DefaultContextWindowIndex'&keyword='jlca1003'"
		private static readonly byte[] EMPTY_BYTE_ARRAY = new byte[0];
		
		/// <summary> Extract a header from a packet.
		/// 
		/// </summary>
		/// <param name="offset">the offset in bytes to the start of the embedded header.
		/// </param>
		/// <param name="headerLen">the length of the header embedded in the packet.
		/// </param>
		/// <param name="bytes">the packet data, including the embedded header and data.
		/// </param>
		/// <returns> the extracted header data.
		/// </returns>
		public static byte[] extractHeader(int offset, int headerLen, byte[] bytes)
		{
			// null in = null out ?
			if (bytes == null)
				return null;
			// negative in, empty array out
			if ((offset < 0) || (headerLen < 0))
				return EMPTY_BYTE_ARRAY;
			
			// verify that requested length is in the bounds of the array
			int useLen = (headerLen <= (bytes.Length - offset)?headerLen:(bytes.Length - offset));
			// verify that requested offset is also in the bounds
			if (useLen <= 0)
				return EMPTY_BYTE_ARRAY;
			
			byte[] header = new byte[useLen];
			Array.Copy(bytes, offset, header, 0, useLen);
			return header;
		}
		
		/// <summary> Extract data from a packet.
		/// 
		/// </summary>
		/// <param name="offset">the offset in bytes to the start of the embedded header.
		/// </param>
		/// <param name="headerLen">the length of the header embedded in the packet.
		/// </param>
		/// <param name="bytes">the packet data, including the embedded header and data.
		/// </param>
		/// <returns> the extracted packet data.
		/// </returns>
		public static byte[] extractData(int offset, int headerLen, byte[] bytes)
		{
			// null in = null out ?
			if (bytes == null)
				return null;
			// negative in, empty array out
			if ((offset < 0) || (headerLen < 0))
				return EMPTY_BYTE_ARRAY;
			
			int dataLength = bytes.Length - headerLen - offset;
			
			// check that requested datalength is valid.
			// (it may not be if packet values are invalid.)
			if (dataLength <= 0)
				return EMPTY_BYTE_ARRAY;
			
			// valid length, go for it dude
			byte[] data = new byte[dataLength];
			Array.Copy(bytes, offset + headerLen, data, 0, dataLength);
			return data;
		}
		
		/// <summary> Extract data from a packet.
		/// 
		/// </summary>
		/// <param name="offset">the offset in bytes to the start of the embedded header.
		/// </param>
		/// <param name="headerLen">the length of the header embedded in the packet.
		/// </param>
		/// <param name="bytes">the packet data, including the embedded header and data.
		/// </param>
		/// <returns> the extracted packet data.
		/// </returns>
		public static byte[] extractData(int offset, int headerLen, byte[] bytes, int dataLength)
		{
			// null in = null out ?
			if (bytes == null)
				return null;
			// negative in, empty array out. request for no-data in, empty array out
			if ((offset < 0) || (headerLen < 0) || (dataLength <= 0) || ((offset + headerLen) > bytes.Length))
				return EMPTY_BYTE_ARRAY;
			
			//-- make sure dataLength + offset + headerLen <= bytes.length
			if ((dataLength + offset + headerLen) > bytes.Length)
			{
				//-- adjust dataLength
				dataLength = bytes.Length - headerLen - offset;
			}
			
			//-- valid length, go for it dude
			byte[] data = new byte[dataLength];
			Array.Copy(bytes, offset + headerLen, data, 0, dataLength);
			return data;
		}
	}
}